Emphasis to this point has been placed on exploring the role of fibronectin in the binding and ingestion of opsonized particles by monocytes. Evidence has been obtained that fibronectin enhances the binding and ingestion of IgG coated erythrocytes and of C3b coated erythrocytes. The augmentation of ingestion by fibronectin is more striking than its augmentation of binding. The fibronectin acts directly on monocytes, rather than interacting with the IgG or complement coated particle. Fibronectin acts through a trypsin-sensitive receptor on monocytes. Polymorphonuclear leukocytes and lymphocytes do not have this receptor. ELISAs for both guinea pig and human fibronectin have been developed with a sensitivity of less than 30 ng/ml. These assays have been used to measure fibronectin levels in patients with systemic mastocytosis, cold urticaria, and hereditary angioedema.